This invention relates generally to migration imaging, and more specifically to an improved migration imaging member and a process for using the improved migration imaging member.
Dry migration imaging members have been extensively described in the patent literature, for example, in U.S. Pat. No. 3,909,262 and U.S. Pat. No. 3,975,195, the disclosures of both being incorporated herein in their entirety. In a typical embodiment of a migration imaging system, a migration imaging member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material was originally in the form of a fracturable layer contiguous the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer.
The expression "softenablen" as used herein is intended to mean any material which can be rendered more permeable thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of the migration marking material is accomplished by dissolving, swelling, melting or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer may be particulate, semi-continuous, or microscopically discontinuous in various embodiments of the migration imaging members of the present invention. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members of the inventive system.
The expression "contiguous" as used herein is intended to mean in actual contact; touching; also, near, though not in contact; and adjoining, and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis, the surface of the softenable layer spaced apart from the substrate.
The expression "adjacent" as used herein is intended to mean in actual contact; touching; also, near, though not in contact; and adjoining, and is intended to generically describe the relationship of one or more layers overlying the surface of the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the image on any original that was employed.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the image on the original and the light areas of the image formed on the migration imaging member correspond to the dark areas of the image on any original that was employed.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this application by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [lo/l]
where l is the transmitted light intensity and lo is the incident light intensity. For the purpose of this invention, the value of transmission optical density includes the substrate density of about 0.2 which is the typical density of a metallized polyester substrate. While contrast density is measured by diffuse densitometers in this application, it should be noted that measurement by specular densitometers gives substantially similar results.
There are various other systems for forming such images, where non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patents, which also discloses a variety of methods which may be used to form latent images upon migration imaging members.
Various means for developing the latent images may be used for migration imaging systems. These development methods include solvent wash-away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash-away or meniscus development method, the migration marking material in the light-struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore the image-sense of the developed image is optically sign-reversed, i.e. positive to negative or vice versa. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the other previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being selffixed because the marking material particles are trapped within the softenable layer. In the heat, or vapor softening developing modes, the migration marking material in the light-struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6-0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. This region thus exhibits maximum optical density (D.sub.max). Therefore, the image sense of the heat or vapor developed images is optically signretaining, i.e. positive-to-positive or negative-to-negative.
Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. Such a technique is described, for example, in U.S. Pat. No. 3,795,512.
One common migration imaging member comprises a metalized plastic substrate bearing a softenable layer. For some demanding commercial applications, it is essential that the structural integrity and imaging properties of migration imaging members must be ensured under all practical usage conditions. Many prior art migration imaging members fail to meet these requirements because of poor mechanical properties of the softenable materials. For example, many migration imaging members, such as those containing a 80:20 mole ratio styrene-hexylmethacrylate copolymer in the softenable layer exhibit poor adhesive properties such that the softenable layer delaminates from the substrate when the members are rigorously handled during use. Additionally they provide insufficient blocking resistance in that blocking occurs under conditions of elevated temperatures and pressure, e.g. when the members are tightly wound in a roll during manufacture, storage or use. Furthermore some prior art softenable materials provide only inferior imaging characteristics because of improper viscoelastic properties, especially when heat only is used to develop the migration imaging member.
A xeroprinting master plate may be prepared from migration imaging members by uniform negative corona charging of the migration imaging members, imagewise exposure, followed by heating. The resulting imaged migration imaging member may then be used as a xeroprinting master. Xeroprinting is performed by uniform corona charging and uniform exposing the xeroprinting master to activating electromagnetic radiation; this yields an electrostatic latent image on the master; the electrostatic image is toned and the toner image is transferred and fused to paper to produce a print. The uniform charging, uniform exposure, toning, transfer and fusing steps may be repeated to produce multiple prints. To be useful as a xeroprinting master, it is necessary that the softenable layer of migration imaging members retain charge transport materials, such as the charge transport materials disclosed in U.S. Pat. No. 4,536,458 issued to Dominic S. Ng and U.S. Pat. No. 4,536,457 issued to Man C. Tam, in the softenable layer over many imaging cycles in order to provide imagewise photodischarge and charge transport capabilities to the xeroprinting master. For some commercial printing applications demanding high quality and resolution such as in color proofing and printing, xerographic liquid toner is generally the preferred developer material because of the smaller toner particle size which produces superior resolution prints compared with the relatively largersized dry toners. Conventional xerographic liquid developers typically comprise a liquid carrier having marking particles dispersed therein. However xeroprinting masters prepared from prior art migration imaging members, for example, those containing a 80:20 mole ratio styrenehexylmethacrylate copolymer and N,N'-diphenyl-N,N'-bis(3"-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine charge transport molecules in the softenable layer, suffer a severe loss of their charge transport properties when contacted with such liquid developers. This is because the charge transport molecules leach out of the softenable layer during contact with the liquid carrier component of liquid developers. This results not only in contamination of the liquid developers but also in rapid degradation of the charge transporting properties, and eventually the xeroprinting capability of the xeroprinting master during cycling. Additionally, in order to produce high quality prints of adequate optical density, the electrostatic contrast potential must be sufficiently high to provide a developable electrostatic latent image. The requirement of high electrostatic contrast potential necessitates the use of a relatively thicker softenable layer in the imaging members. However, because of rather poor adhesion to the conductive substrates, such as aluminized polyester, of many otherwise excellent softenable materials for migration imaging members, attempts to increase the thickness of the softenable layer to obtain sufficient electrostatic contrast potential often result in film delamination either during manufacture, storage or use.